1. Field of the Invention
The invention relates to the production of nitrogen from air. More particularly, it relates to the production of dry, high purity nitrogen.
2. Description of the Prior Art
High purity nitrogen is desired for many chemical processing, refinery, metal production and other industrial applications. While various techniques are known for the production of nitrogen by air separation, pressure swing adsorption (PSA) processing is particularly desirable for relatively small sized operations in which the use of a cryogenic air separation plant may not be economically feasible.
In the PSA process for air separation, feed air is passed at a higher adsorption pressure to an adsorbed bed capable of selectively adsorbing either nitrogen or oxygen as the more readily adsorbable component of air. The bed is thereafter depressurized to a lower desorption pressure for desorption of said more readily adsorbable component and its removal from the bed prior to the introduction of additional quantities of feed air to the bed as cyclic adsorption desorption operations are continued in the bed. The PSA process is commonly carried out in multi-bed systems, with each bed employing the desired processing sequence on a cyclic basis interrelated to the carrying out of said processing sequence in other beds in the system.
Two different PSA processes and systems have been employed commercially to produce product nitrogen at purities of up to about 99.5%. In one approach, a rate selective carbon molecular sieve adsorbent is used in a fast processing cycle, based on the selective adsorption of oxygen as the more readily adsorbable component of air, to produce nitrogen, as the less readily adsorbable component withdrawn from the product end of the bed, at the adsorption pressure and with a relatively low dewpoint, such as -40.degree. F. It has been recognized, however, that the presence of moisture in the feed air to such a PSA system significantly reduces the separation efficiency of the adsorbent beds. For this reason, it is common practice to employ a separate PSA adsorbent dryer ahead of the air separation PSA system to remove moisture from the feed air before it passes to said air separation PSA system.
In another PSA process and system approach, adsorbents capable of selectively adsorbing nitrogen from air on an equilibrium selective basis are employed. In such systems, air is passed to the adsorbent bed, typically at slightly above atmospheric, and vacuum pumps are used to draw off a nitrogen-rich wet nitrogen product stream from the bed. Zeolitic molecular sieves are commonly employed in such operations. The Werner et al. patent, U.S. Pat. No. 4,599,094, discloses the details of the processing sequence used in such a PSA nitrogen process for the recovery of high purity nitrogen product. The nitrogen product obtained is generally wet since, in addition to the moisture transferred from the inlet air to the product nitrogen, some additional moisture is commonly added thereto from the water seals of the vacuum pumps. In many such applications, therefore, it is necessary to compress the recovered nitrogen product and remove moisture therefrom. This is done to prevent condensation and subsequent corrosion or freezing in plant piping and instrumentation, or because its presence is incompatible with the desired end use of the product nitrogen. One feasible approach to the problem of moisture in the product nitrogen is to use a product compressor, aftercooler, moisture separator and adsorptive dryer to produce a dry, high purity nitrogen stream.
The use of adsorptive pre- or post-dryers in PSA-nitrogen systems adds significantly to the overall complexity and cost of the process and system, and reduces its reliability. Such dryers typically have multiple adsorbent vessels with interconnecting piping and valves. A significant amount of nitrogen product gas, e.g. from 5 to 30% of the feed air, may be required for bed regeneration purposes as part of the overall PSA processing sequence employed. If a thermal swing cycle were used, some purge energy expenditure would also be required. If the waste oxygen stream were used as a bed regeneration gas, special precautions efficiency and cost of the nitrogen production operation, there is a desire in the art for improvements in the production of dry, high purity nitrogen by the PSA approach, particularly improvements with respect to the removal of moisture from high purity nitrogen.
It is an object of the invention, therefore, to provide an improved process bed system for the production of dry, high purity nitrogen product.
It is another object of the invention to provide an improved process and system utilizing the PSA approach for air separation and providing for desired pre- or post-drying for moisture removal and the recovery of dry, high purity nitrogen product.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.